Photographic processes using silver halides have, in the past, been most widely used. They are used because of their excellent photographic properties such as sensitivity and gradation control as compared with other photographic processes, such as an electrophotographic process or a diazo photographic process. In recent years, substantial research has been done with respect to image formation processes for photographic materials using silver halide. As a result of this research many techniques capable of obtaining images with ease and rapidly have been developed by changing the conventional wet process using a developing solution into a dry process such as a process using heat.
Heat-developable photographic materials are known in the field of these techniques, and heat-developable photographic materials and processes therefor have been described in U.S. Pat. Nos. 3,152,904, 3,301,678, 3,392,020 and 3,475,075, British Pat. Nos. 1,131,108 and 1,167,777, and Research Disclosure, pp. 9-15, No. RD-17029, June, 1978.
Many different processes for obtaining color images have been proposed. With respecct to processes for forming color images by the reaction of an oxidation product of a developing agent with a coupler, it has been proposed to use a p-phenylenediamine type reducing agent and a phenolic coupler or an active methylene coupler as disclosed in U.S. Pat. No. 3,531,286, a p-aminophenol type reducing agent as disclosed in U.S. Pat. No. 3,761,270, a sulfonamidophenol type reducing agent as disclosed in Belgian Pat. No. 802,519 and Research Disclosure, pp. 31 and 32, September 1975. In addition, the combination of a sulfonamidophenol type reducing agent and a 4-equivalent coupler has been disclosed in U.S. Pat. No. 4,021,240. These processes, however, are disadvantageous in that turbid color images are formed due to the simultaneous formation of a reduced silver image and a color image on the exposed area after heat development. A proposed process for eliminating these disadvantages comprises removing silver images by liquid processing or transferring only the dyes to another layer, for example, a sheet having an image receiving layer. However, it is not easy to transfer only the dyes as distinguishable from unreacted substances.
A process which comprises introducing a nitrogen containing heterocyclic group into a dye, forming a silver salt and releasing a dye by heat development has been described in Research Disclosure, pp. 54-58, No. RD-16966, May, 1978. According to this process, clear images cannot be obtained, because it is difficult to control the release of dyes from unexposed areas. Therefore, it is not a conventional process.
Furthermore, with respect to a process for forming a positive color image by a thermal silver dye bleach process, useful dyes and methods for bleaching have been described, for example, in Research Disclosure, pp. 30-32, No. RD-14433, April, 1976, ibid., pp. 14-15, No. RD-15227, December, 1976, U.S. Pat. No. 4,235,957, etc.
However, this process requires an additional step and an additional material for accelerating bleaching of dyes, for example, heating with a superimposed activating agent sheet. Furthermore, it has a drawback that the resulting color images are gradually reduced and bleached by coexisting free silver during preservation for a long time.
A process for forming a color image utilizing a leuco dye has been described, for example, in U.S. Pat. Nos. 3,985,565 and 4,022,617. However, with this process it is difficult to stably incorporate the leuco dyes in the photographic material and coloration gradually occurs during preservation.